A credit card is required when you have more than 3 active users. If you are on the business plan then your credit card will get billed each month depending on how many users you have. In the below screen you can enter the amount of users you have, then you can add or update your credit card.
Later when you add or archive users, make sure to go back here and adjust the number of users you need up or down. Then after you have made changes your credit card will be billed a pro-rated amount if you are adding users.
Menu SetupSettings Application Billing tab Subscription Plans tab